Problems orgasming

Ok, I'm an eightteen year old college male and I came home this weekend and my girlffriend came home as well from her college. We have not seen each other in a month and were happy to see each other. At the end of the first night, I had seen to it that she had enjoyed herself and then I was myself unable to orgasm. We have done this several times before, and never had any problems. We were both virgins when we began, no stds, and it is very frustrating and disappointing. She feels sexually indadequate and upset and this is hard to see and let happen. The second night wasn't any better and the same thing occured. We had tried several times both nights and nothing would work. I can get and stay erect, we both have the desire, and nothing seems to work. Is it a medical problem? There are no problems between us and we're both very comfortable around one another. I was reading several sites and nothing seems to offer any help. Please help and offer insite.